In various industries it is necessary to clean oils from the surfaces of metal prior to decorating such surfaces with paint, ink or various other protective or decorative coatings. For example, in the manufacture of metal containers, such as beverage containers, metal blanks are stamped and then formed by drawing and ironing into a desired container configuration, and finally decorated. During the forming process, lubricants and coolants are deposited on the metal surfaces. Residual quantities of lubricants and coolants must be removed from the surface of the metal prior to printing, overcoating, internal coating or other decorating or detailing of the cleaned metal surface.
Currently, the industry uses cleaning agents consisting of either relatively strong acid or caustic solutions. Primarily strong acid solutions consisting of hydrofluoric, sulfuric, phosphoric, nitric, and similar acids are used. For example, U.S. Pat. Re. No. 32,661 to Binns discloses a method of cleaning aluminum using an acidic aqueous cleaning solution having a pH less than 2. Similarly, U.S. Pat. No. 3,969,135 to King discloses a composition and process for cleaning aluminum using a low temperature aqueous acidic solution with a pH of less than 2.
While the use of acidic and caustic solutions to clean metal is effective to remove oils, the use of such solutions presents multiple problems in the treatment of used cleaning and rinse solutions, and has negative effects on the appearance and integrity of such metal when it is used, for example, in the manufacture of containers. The acidic and caustic cleaning agents currently in use in container manufacturing operations etch and pit the metal surfaces being cleaned, thereby diminishing the strength of the metal, often making such containers unusable. During drawing and ironing procedures for the manufacture of aluminum beverage containers, manganese-iron particles in the aluminum may create pin holes and gouges in the containers. Acidic and caustic cleaning agents deepen such holes and gouges and often result in containers deemed unusable "pinhole leakers."
The etching and pitting of metal surfaces caused by use of an acid and caustic wash cleaning agent increases the roughness of the metal surface and consequently reduces the reflectivity of the metal surface. Reduced reflectivity in turn results in reduced color enhancement of inks applied to the metal surface. The reduced reflectivity and reduced color enhancement are aesthetic detractors and, therefore, metal surfaces cleaned with acidic or caustic washes are not as pleasing in appearance to the ultimate consumer. Such aesthetic detraction applies not only to aluminum beverage containers, but also to a myriad of other products in the marketplace, including, for example, finished metal surfaces for cars, trucks and airplanes, face plates for calculators and computers, etc.
Metal containers cleaned with an acidic or caustic wash require more ink, overcoat and inner coat because the etches and pits resulting from such washes result in a greater surface area to be covered. Reduction in the etching and pitting on a container's surface would result in the use of less ink, overcoat or inner coat and, therefore, in a significant cost savings.
The use of acidic or caustic washes also creates ionization points on a metal's surface. These ionization points form electrical attractions to polar molecules, such as water. Typically, after cleaning of a metal surface with an acidic or caustic wash, the metal surface is rinsed with water and then dried. Because rinse water tends to adhere to the ionization points formed on the metal's surface, a longer drying time is required.
Acidic and caustic cleaning agents currently used to clean metal surfaces also pose serious health risks to personnel performing cleaning operations.
Finally, substantial environmental problems are created due to the use of acidic and caustic agents in the metal cleaning industry. Conventional acid or caustic cleaning agents require neutralization prior to their discharge. Several disadvantages are encountered in such neutralization, including the cost of the materials and the handling problems associated with such toxic chemicals. The corrosive nature of acid and caustic cleaning solutions also damages processing equipment used in the cleaning of metal containers. Moreover, once mixed with rinse water, acidic and caustic cleaning agents are not readily recoverable. Further, the acidic and caustic cleaning agents currently in use etch the metal surfaces and produce waste solutions with dissolved metal, creating additional waste disposal problems.
The use of conventional acidic and caustic cleaning agents also reduces the mobility of containers conveyed in a contiguous manner on a production line due to the friction between the containers created by etches and pits on the container's surfaces. The reduced mobility of the containers on a production line results in jamming of the processing machinery and causes downtime on the production line. In a typical container manufacturing facility, acid washed containers typically jam the production line machinery approximately one to three times an hour. Such jamming necessitates the stopping of the production line and the manual extraction of containers which fall out of the production track.
In view of the above, a need exists for a new method for cleaning and finishing metal surfaces, especially in the manufacturing of metal containers. The present invention addresses this need by providing a method for manufacturing metal containers using a terpene based cleaning agent which has a substantially neutral pH and is capable of removing lubricants and coolants from metal surfaces without the problems associated with the use of acidic and caustic cleaning agents. The present invention also provides an economical method of recycling the cleaning agent, lubricants, coolants and water used in container manufacturing processes in a substantially closed loop system.